Half-Round Trim on Armor
It's a bit hard to tell what you want to do, but It sounds like you want to make a round &quot;border&quot;. In that case, Beveling is the tool for the job.
Select an edge that you need to round, and press CtrlB. Move your mouse to control the depth of the bevel, and scroll to adjust the segments of the bevel, sort of like its resolution. There are a lot of settings that you can change. You can even bevel multiple edges at the same time by selecting them all and doing the same thing. Additionally, there is a bevel modifier that can bevel every single edge.